St. Lawrence Basin and the Great Lakes. 235 
bounded, especially upon the southern side, by high and 
precipitous escarpments, some of which are submerged. 
But upon their northern sides there are also lesser vertical 
escarpments, now submerged, with walls facing the old 
valley. The valley was excavated when the continent was 
at a high altitude, for the eastern portion stood at least 1,200 
feet higher than at present, as shown by the channels in the 
lower St. Lawrence, in Hudson’s straits, and in the New 
York and Chesapeake bays. The valley was obstructed in 
part by drift, and in part bya north and northeastward 
differential elevation of the earth’s surface, due to internal 
movements. The measurable amount of warping defied 
investigation until recently, but now it is measured by the 
amount of uplift of beaches and sea cliffs. Only one other 
explanation of the origin of the basins has been given—the 
“Erosion by Glaciers.” (a) Because the latter occur in 
glaciated regions. (b) That the glaciers are considered (by 
some) to erode. (c) The supposed necessity, as the terres- 
trial warping was not known. 
In reply: Living glaciers abrade, but do not erode, hard 
rocks, and both modern and extinct glaciers are known to 
have flowed over even loose morainesand gravels. Again, 
even if glaciers were capable of great plowing action, they 
did not affect the lake valleys, as the glaciation of the sur- 
face rocks shows the movement to have been at angles (from 
15° to 90°) to the direction of the side of the vertical escarp- 
ments against which the movement occurred. Also the 
vertical faces of the escarpments are not smoothed off, as are 
the faces of the Alpine valleys, down which the glaciers 
have passed, Lastly, the warping of the earth’s surface in 
the lake region, since the beach episode, after the deposit of 
the drift proper, is sufficient to account for all rocky barriers 
which may obstruct the basins. 
